1. Field of the Invention
The present invention generally relates to a driving mechanism, and particularly relates to the driving mechanism using an electromechanical transducer, such as a piezoelectric element, for driving a movable object, the driving mechanism being preferably used for accurately driving an X-Y driving table, a photographing lens of a camera, a probe of a scanning type of tunneling electron microscope, and so on.
2. Description of the Related Arts
Conventionally, there has been proposed a driving mechanism which achieves a high resolution on sub-.mu.m order by using a piezoelectric linear actuator, instead of using a stepping motor.
For example, a driving mechanism 110 the main part of which is shown in an exploded perspective view of FIG. 1 and in an assembling perspective view of FIG. 2, comprises a fixing member 124 which is fixed to an unshown base, a piezoelectric element 122, a driving shaft 126 which is slidably supported by the fixing member 124, and a driving unit 128 which is coupled to an unshown member to be driven such as a stage or the like on which some other parts are mounted.
The piezoelectric element 122 is a kind of electromechanical transduction element or electromechanical transducer, and changes in length when a voltage is applied thereto. One end face 122a in a direction of expansion and contraction of the piezoelectric element 122 is fixed to the fixing member 124, and the other end face 122b in the direction thereof of the piezoelectric element 122 is fixed to one end 126a of the driving shaft 126. The driving unit 128 frictionally engages the driving shaft 126.
According to this driving mechanism 110, when some voltage in a saw-toothed wave form of a periodic pulse, for example, is applied to the piezoelectric element 122, the driving shaft 126 is reciprocated in the axial direction thereof, causing the driving unit 128 to be moved along the driving shaft 126.
In respect of such a driving mechanism 110, a longer driving shaft 126 is required in order to widen a movable range of the member to be driven, namely, in order to realize a longer stroke of the driving unit 128.
However, if the driving shaft 126 is longer, the mass of the driving shaft 126 increases, thus the responsivity of the piezoelectric element 122 deteriorating. As a result, the driving shaft 126 can not be driven at high frequencies, and this incurs a slowdown in moving speed of the member to be driven. Namely, with the mechanism, it is difficult to realize a longer stroke of the member to be driven while the moving speed of the member to be driven is maintained unchanged.
On the other hand, there has been proposed a stage using displacement of the piezoelectric element itself. However, with such a stage, it was difficult to realize enough displacement.
On the other hand, there has been proposed a driving mechanism which realizes a very high resolution and a long stroke with a self-moving piezoelectric linear actuator, namely, with an impact type of actuator, as shown in the schematic diagrams of FIGS. 3A to 3D.
More specifically, this driving mechanism 100 is constructed as follows. As shown in FIGS. 3A to 3C, one end face 106a of a movable member 106 which is coupled to an unshown member such as a stage to be driven, is fixed to one end in a direction of expansion and contraction of a piezoelectric member 102. The other end in the direction thereof of the piezoelectric member 102 is fixed to a member of inertia 104. The movable member 106 is placed on a support surface 108. A bottom surface 106b of the movable member 106 frictionally engages the support surface 108, causing a frictional force to be generated therebetween.
This driving mechanism 100 is operated as follows. That is, when some voltage in a saw-toothed wave form of a periodic pulse, for instance, as shown in FIG. 3D is applied to the piezoelectric element 102, the piezoelectric element 102 slowly expands and the member of inertia 104 moves from the state shown in FIG. 3A to the state shown in FIG. 3B. Next, the piezoelectric element 102 rapidly contracts or shrinks so that the movable member 106 is moved from the state shown in FIG. 3B to the state shown in FIG. 3C by an impact of the member of inertia 104. Along with this movement of the movable member 106, the member to be driven by the movable member 106 is also driven and moved.
According to the driving mechanism 100, the movable member 106 moves on the support surface 108. Therefore, by lengthening the support surface 108, the stroke of the member to be driven can be lengthened to any extent, theoretically.
However, in this driving mechanism 100, if the member of inertia 104 is made heavier for the purpose of realizing increasing speed thereof with a greater impact, the responsivity of the piezoelectric element 102 becomes worse, thus resulting in a lower speed thereof to the contrary. Meanwhile, if the member of inertia 104 is made lighter for the purpose of realizing increased responsivity of the piezoelectric element 102, the impact becomes smaller, thus resulting in a lower speed thereof as well.
That is, it is a difficult matter to determine the mass of the member of inertia 104, and there is a limitation in achieving higher speed thereof with this type of driving mechanism 100 using the impact actuator.
Also, this driving mechanism 100 is so constructed that the movable member 106 and the member to be driven are guided separately and independently of each other. As a result, there arises a problem that the member to be driven can not be driven with a sufficient parallelism between the driving direction of the movable member 106 and the guiding direction in which the member to be driven thereby is guided.